Hook 400 lbs on the TPH and ideally you should have 400 hanging off the front. This of course ignores torque moment and a lot of other "center of gravity" (CG) calculations, including the height of the weight above the CG.
The idea is to keep the machine in balance on it's feet to help avoid tipping.
Most folks simply use what they have at hand. As a practical matter wheel weights only offset loads applied to the opposite end of the machine. In other words, rear wheel weight offsets the loader and it’s payload, but does nothing to offset TPH attachments because both weights are on the same side of the CG.
The loader, front wheel weight, and suitcase weights, all act to offset TPH attachment weight, because they are on opposite sides of the CG.
Think of it this way –
The CG of most tractors is somewhere near the instrument panel a few inches above the frame, somewhere near your ankles. This will vary depending on your set up, but in all cases the CG MUST be kept inside a box defined by the four wheels on the tractor and at the height of the CG. If the CG is allowed to get outside that box the tractor will tip over. It is also a good idea to keep the CG as low as possible to avoid tipping on uneven ground. If possible it is best to keep the CG so low that it is actually at or just below the axel height. If you can accomplish this, the machine can operate on much steeper grades and still keep the CG inside the box. Since tractor wheels are different diameters the axels are at different heights, and the CG is usually located somewhere between the axels vertically.
If you draw a vertical line through the CG, this is the direction that gravity will act on the tractor. That line is always vertical. The box defined by the wheels is always tipping in relation to that vertical line. If at any time the line gets outside the box defined by the four wheels, your going over. You can feel this when you carry a load too high and things get a little iffy. In fact that is what is happening when you lift too much weight in the loader, the CG moves out ahead of the front axel and the machine tips forward.
Almost the same thing with towing. If you put the tow hook above the rear axel, when you take a strain you are effectively raising the CG above the axel and pulling the gravity line off vertical and outside the box to the rear and the front wheels lift off. Pull from below the axel and you move the CG out of the box just as before but this time it is below the axel and the front of the tractor is forced down. It can’t go that way because the planet is in the way so you are ok. You are in effect countering the weight with the ground at the front wheels.
But you know all that so back to the original question.
If you add 400 pounds to the tractor, 6 feet in front of the CG, you would theoretically want to add 400 pounds 6 feet behind the CG to offset it. This works for height as well, add 400 pounds above the CG and you would want to add AT LEAST 400 pounds below as well. You want to add both weights as low as possible to keep from raising the CG. Because the more difference there is between the height of the CG and the box, the tipper the arrangement gets.
Now the practical effects of all this can be seen quite readily on a tractor. The CG is in constant motion when the tractor is working. Especially when working with the loader. The vertical gravity lone is also in constant motion. Since the situation is not static, most people find it satisfactory to add sufficient weight on the rear to compensate for the loader itself and at least a portion of the load it will carry. A certain amount of “rear” heavy is accepted in the arrangement under no load conditions. Since this varies constantly, it is almost impossible to state an exact weight to add.
Most of us get a “feel” for what to add and where. Sometimes we guess wrong and our mistake will appear in the Safety discussion on TBN. The manufacturers know all of this and that is why they invented the ROPS.
